This invention relates to improvement of the loudspeaking function of key telephone system.
FIG. 1 shows configuration of a key telephone according to the prior art. A block designated reference number 1 is a main apparatus which is connected to a plurality of key telephones 2.sub.1 - 2.sub.n through two-wire speech signal lines 3.sub.1 -3.sub.n and through two-wire control signal lines 4.sub.1 -4.sub.n. In the main apparatus, the speech signal lines 3.sub.1 -3.sub.n are connected through speech signal transformers 5.sub.1 -5.sub.n, two/four wire conversion circuits 6.sub.1 -6.sub.n and CODECs 7.sub.1 -7.sub.n to a time switch 8. Also, the control signal lines 4.sub.1 -4.sub.n are connected through control signal transformers 9.sub.1 -9.sub.n and two/four wire conversion circuits 10.sub.1 -10.sub.n to a control circuit 11. The time switch 8 performs speech pat switching under control of the control circuit 11.
In the key telephone 2, the speech signal line 3 is connected through a speech signal transformer 12 and a two/four wire conversion circuit 13 to mode selector switches 14 and 15. The mode selector switches 14 and 15 select a handset conversation mode or a loudspeaking conversation mode. When contacts a of the switches are on, the handset conversation mode is selected. In the handset conversation mode, a handset 16 is connected to the two/four wire conversion circuit 13 through a received speech signal amplifier 17 and a transmitted speech signal amplifier 18. On the other hand, in the loudspeaking conversation mode in which contacts b are on, a loudspeaking circuit 19 is connected to the two/four wire conversion circuit 13. Received speech signals from the two/four wire conversion circuit 13 are sent through a received voice switching circuit 20 and a speaker amplifier 21 to a speaker 22, and transmitted speech signals from a microphone 23 are sent through a microphone amplifier 24 and a transmitted voice switching circuit 25 to the two/four wire conversion circuit 13. At the same time, the levels of the received speech signals and transmitted speech signals are detected with a received speech detector 26 and a transmitted speech detector 27, respectively, and then supplied to a comparator 28. The comparator 28 compares the received speech signal level, the transmitted speech signal level and a voltage level from a voltage setter 29, and according to the comparison results of the three levels, reciprocally changes losses of the received voice switching circuit 20 and the transmitted voice switching circuit 25. As a result, a loop gain of a closed loop formed by acoustic coupling between the speaker 22 and the microphone 23 is controlled to be less than 1, and howling (oscillation) can be prevented.
One of the problems in the prior-art key telephone system is that the losses of the received voice switching circuit 20 and the transmitted voice switching circuit 25 in the loudspeaking conversation mode must be set to be greater to prevent the howling because there are a plurality of closed loops which may generate howling. Assume that, for example, in FIG. 1, conversation is held between the key telephone 2.sub.1 in the loudspeaking mode and another separate key telephone 2.sub.n. When the separate telephone 2.sub.n is in the handset conversation mode, there are two closed loops which may generate howling. One of the loops is formed by the following processes: the transmitted speech signal from the microphone 23.sub.1 is sent to the two/four wire conversion circuit 13.sub.1 and a part of the sent signal is mixed as a side tone with the received speech signal, and this side tone is sent to the speaker 22.sub.1, then a part of the side tone is supplied again to the microphone 23.sub.1 owing to acoustic coupling. The other loop is formed through the following processes: the transmitted speech signal from the microphone 23.sub.1 is sent to the two/four wire conversion circuit 6.sub.n in the main apparatus and a part of the sent signal is mixed as a side tone with the transmitted speech signal from the telephone 2.sub.n, and this side tone is returned to the telephone 2.sub.n, and then a part of the side tone is fed from the speaker 22.sub.1 to the microphone 23.sub.1 again. When the accompanying telephone 2.sub.n is also in the loudspeaking conversation mode, further another closed loop is added. The loop is formed through the following processes: the transmitted speech signal from the microphone 23.sub.1 is sent to the speaker 22.sub.n and a part of the sent signals is supplied to the microphone 23.sub.n owing to acoustic coupling; the signals supplied to the microphone 23.sub.n are sent to the speaker 22.sub.1, then a part of the sent signals is supplied again to the microphone 23.sub.1.
Because there may be two or more such closed loops as described above, the losses of the voice switching circuits 20 and 25 must be set so large that each loop gain does not exceed 1. As a result, a high voice level is required for switching between the speech signal transmission state (where the loss of the received voice switching circuit 20 is large and the loss of the transmitted voice switching circuit 25 is small) and the speech signal reception state (where the loss of the received voice switching circuit 20 is small and the loss of the transmitted voice switching circuit 25 is large), thus causing the problem of difficulty in transmitting voices mutually.